1. Field of the Invention
The present invention relates to target plates for use in converting information-carrying thermal radiation to storable or visual form, and in particular to such plates which have application in a thermal vidicon.
2. Description of the Prior Art
Several types of devices operating according to different physical principles are known for converting information-carrying thermal radiation into storable or visual form. Such devices have in common the utilization of a physical conversion effect which transforms impinging thermal radiation into a locally distributed pattern which produces an image corresponding to the location and intensity of the thermal radiation.
The material in which the physical conversion effect takes place is generally referred to as the conversion layer, and in many devices the conversion layer takes the form of a plate on which thermal radiation impinges. In many applications, the conversion layer or conversion plate is comprised of pyroelectric material which generates a charge image consisting of locally distributed electrical charge which charge image is then utilized to generate a visual or storable image by scanning the surface of the plate with an electron beam. Other devices make use of a thermally generated voltage pattern or gradient opposite a base electrode, or make use of the thermoresistive effect to generate a locally distributed pattern of electrical resistance. Finally, some devices utilize the impinging radiation to generate a pattern of varying optical indices of refraction in an appropriate layer adjacent to the conversion plate. The particular physical effect and structure utilized to generate the visual or storable image may be selected depending upon individual needs and conditions associated with a particular application.
Each of the above-described conventional structures for converting thermal radiation into visual or storable form has a limited resolution capability resulting from the lateral transfer of thermal radiation within the conversion layer. By lateral transfer is meant radiation of thermal energy in a direction in the conversion layer perpendicular to the direction of the impinging thermal radiation. This transfer results from the heat transfer properties inherent in the conversion layer, which heat transfer proceeds according to an exponential function. In order to achieve a recognizable or storable image, two competing physical effects must be accommodated. First, the conversion layer must be exposed to the impinging thermal radiation for a sufficient amount of time so that the associated temperature gradient can be generated within the conversion layer to produce an image of sufficient intensity. This is contrasted with the fact that the sharpness of the image so generated is directly determined by the lateral thermal transmission within the conversion layer which lateral thermal transmission increases with increased exposure to the impinging radiation. Best resolution is obtained when the discrete local distributions of thermal conversion exist only for a brief time, so that the discrete distributions are not permitted to merge due to lateral thermal transmission and thereby blur the composite image.
A conversion plate structure for decreasing the lateral thermal transmission and thus increasing the optical resolution of the produced image in devices of the type initially described is disclosed in German OS No. 22 23 288. The structure disclosed therein is a conversion layer in a raster form applied to a supporting substrate, which raster consists of discrete blocks of detector elements of pyroelectric material. The conversion layer thereby consists of a mosaic of pyroelectric elements which are separated from one another by a network of channels. Inductive transmission between the discrete elements is thus reduced, resulting in a more highly resolved composite image. This structure has the disadvantage of requiring a carrier plate to support the individual pyroelectric elements. The carrier plate itself has an unavoidable thermal capacity which decreases the sensitivity of the applied conversion layer. Moreover, mechanical stability is a problem in the structure disclosed in German OS No. 22 23 288. because the discrete pyroelectric elements must be applied to the carrier with some form of supporting film or adhesive material which further impairs the sensitivity and resolution of the raster.